The remaining three northern white rhinos, all elderly, are in a closely guarded preserve in Kenya.

The subspecies has been decimated by poachers, who kill the rhinos for their horns. The horns are in high demand in parts of Asia where some people claim they have medicinal properties for treating everything from hangovers to cancer.

In an effort to preserve the species, the San Diego zoo took possession earlier this month of six female southern white rhinos from South Africa.

Zoo researchers are working on developing northern white rhino embryos to be implanted in the six new arrivals, who will serve as surrogate mothers.

Researchers have said they hope a northern white rhino calf could be born from a San Diego surrogate mother within 10 to 15 years.